139 Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts, Part II

This session is dedicated to studies that illuminate the biotic, geobiological, sedimentological, and geochemical contexts of the evolution of early animal life in the oceans, with a particular focus on the Cambrian Period.

Monday, 20 October 2025
1:30 PM - 5:30 PM
HBGCC, 304B

Uncovering Bias in Acid Maceration of Cambrian Small Shelly Fauna

Paper number: 139-5
Program: Technical Sessions
Time: 2:35 PM – 2:50 PM

Authors

Abstract

The study of Cambrian small shelly fauna (SSF), millimetric remains of a diverse array of animal taxa, frequently relies on destructive chemical extraction techniques, such as acid maceration, to isolate microfossils from host limestones. While effective for recovering fossils with insoluble or secondarily replaced hard parts, acid maceration preferentially dissolves calcareous faunal components, inherently introducing taxonomic and mineralogical biases into the resulting fossil assemblages. Accurately quantifying this bias is critical for refining paleoecological interpretations. 

In this study, we analyzed samples from a measured stratigraphic section of the Third Plain Creek Member, Mernmerna Formation (Cambrian Series 2, Stage 3) in the Ikara-Flinders Ranges of South Australia. Eleven fossiliferous horizons were selected for paired petrographic thin section analysis and acid maceration to characterize SSF before and after extraction. Our methodology combined point counting, manual pixel segmentation, and energy-dispersive X-ray spectroscopy (EDS) to classify SSF by higher-order taxonomic affinity (i.e., faunal abundance) and mineralogical composition (i.e., taphonomic effects). 

We compared the proportion of pre- and post-extraction SSF belonging to each taxonomic group, using Pearson’s correlation tests to evaluate whether the proportion measurable in thin section is a suitable predictor of the proportion found in acid residue. For nearly every group examined, there was no correlation between results. Sponges were the sole exception, with a significant positive correlation between pre- and post-extraction proportions\. Sponges in this assemblage are primarily extracted as silaceous spicules, emphasizing the role mineralogy plays in acid residue sample bias. To test this, the same samples will be next sorted by preservational mode, and similar correlation testing applied. 

This integrated approach reveals methodological biases and enhances our ability to reconstruct more accurate paleoecological and biostratigraphic frameworks. Furthermore, our findings emphasize the need for evaluating microfossil extraction technique suitability across diverse sedimentary facies and fossil preservation modes. 

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413597

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